This invention relates to a countershaft type power transmission and more particularly to an improvement to a countershaft type power transmission as disclosed in the above referenced copending U.S. Pat. Application.
As in the copending U.S. Patent Application, it is proposed to make a gear shifting operation automatically in response to various vehicle operating conditions including a vehicle speed, a throttle opening degree and a position of a control lever.
For making a shift, a clutch, disposed between an engine and a gear box, will be disengaged by a vacuum motor and then a select shaft and a shift shaft are actuated to effect engagement of a desired gear of the gear box by the corresponding hydraulic motors. For engaging a reverse gear, the control lever is disposed at a reverse position, producing a predetermined signal to cause the clutch to be disengaged. After disengagement of the clutch, a hydraulic motor operatively connected to a select shaft rotates the select shaft to one of the plurality of angular positions to select a reverse fork rod and thereafter, another hydraulic motor operatively connected to a shift shaft rotates the shift shaft to one of the plurality of angular positions to move the reverse fork rod in a direction to engage a reverse gear. Since the knob of the control lever must be kept gripped to keep the clutch disengaged, the knob must be released to cause the clutch to reengage or ready for reengagement when the shift has been completed.
The problem with this transmission resides in the fact that gears for the reverse may fail to mesh with each other even if the control lever is moved to the reverse position or may grind with each other upon release of the knob after the control lever has been moved to the reverse position. This occurs when the gears for the reverse are misalinged. Under this condition, even if the control lever is at the reverse position, one of the teeth of the sliding gear on the main shaft will abut with one of the teeth of the reverse gear on the counter shaft, failing to engage in a space between two adjacent teeth of the reverse gear. Accordingly, even if the gears become aligned afterwards, after or upon engagement of the clutch, the gears will not mesh with each other because there is no lost motion connection between the shift shaft and the corresponding motor.